The Head-Up Display (HUD) has been developed from the well-known reflecting gunsight by applying electronic methods to the representation of flight path information, which is optically superimposed on the forward visual field. Previous investigations of the combination of information in superimposed fields are reviewed to show the relationship between properties and design of the HUD. An acceptable standard of safe usage is sought in the evidence of ability to perform concurrent tasks based on display and forward view. Design variables are the position and visible form of the display: constraints are the effects of errors and limitations, as they concern choice of symbol. Position is shown to influence the efficiency with which display and forward view may be used. Pattern is shown to influence learning and tracking accuracy. Errors and limitations affect runway and flight vector symbols adversely, even when used in an auxiliary capacity, but have little effect on nonpictorial elements, or on director and attitude symbols particular conditions. Properties resulting from the application of these results and other rules relating to the design, location, and control of symbols, allow the average user to learn very quickly, and to reach a relatively high level of performance without over concentration. At the same time, the pilot is able to see where he is going, and the transition from instrument to visual flight is virtually eliminated.